1. Field
Exemplary embodiments of the present invention relate to a variable resistance memory device, and more particularly, to a variable resistance memory device having a complementary resistive switch (CRS) structure.
2. Description of the Related Art
Recently, a variety of variable resistance memory devices have been developed, which store data using a material having different resistance states depending on an applied bias voltage. In general, a unit cell of a variable resistance memory device includes a variable resistance material layer interposed between two electrodes for voltage application.
Such a variable resistance memory device may be divided into two types depending on switching characteristics. More specifically, a variable resistance memory device may be switched to a unipolar mode in which set/reset operations are performed using one polarity or a bipolar mode in which set/reset operations are performed using different voltage polarities. The variable resistance memory device in the bipolar mode exhibits uniform switching characteristics and performs a reset operation through an electric field. Here, the variable resistance memory device has a small reset current.
Meanwhile, in order to increase the integration degree of the variable resistance memory device, a so-called cross-point structure has been developed. A cross-point structure includes a plurality of first conductive lines parallel to each other, a plurality of second conductive lines parallel to each other in a direction perpendicular to the first conductive lines, and a plurality of unit cells arranged at the respective intersections between the first conductive lines and the second conductive lines.
In such a cross-point structure, however, since variable resistance material layers are substantially coupled to each other through the first and second conductive lines, leakage current may occur in an unselected cell such as a cell in a low-resistance state.
In order to address the leakage current issues, a variable resistance memory device having a so-called complimentary resistive switch (CRS) structure has been proposed. An exemplary variable resistance memory device has been disclosed in “Complementary resistive switches for passive nanocrossbar memories” (hereinafter, referred to as “Prior Document 1”) published in Nature Materials Volume: 9, pp. 403-406, May 2010.
Referring to Prior Document 1, two structures, each of which has the same structure as a conventional unit cell including a first electrode, a second electrode, and a variable resistance material layer interposed therebetween and operates in a bipolar mode, are stacked symmetrically with each other and are used as a unit cell of the CRS structure. That is, when any one structure in the unit cell of the CRS structure includes the first electrode, the variable resistance material layer, and the second electrode which are sequentially stacked, the other structure includes the second electrode, the variable resistance material layer, and the first electrode which are sequentially stacked. Accordingly, when any one structure is set at the same voltage, the other structure is reset.
In the unit cell of the CRS structure, when a first structure is in a high resistance state (HRS) and a second structure is in a low resistance state (LRS), data ‘0’ is stored, and when the first structure is in the LSR and the second structure is in the HRS, data ‘1’ is stored. Therefore, the unit cell stays in a resistance state over the HRS, regardless of the data stored therein. Therefore, although a variable resistance memory device is implemented with the cross-point structure, a leakage current into an unselected cell does not occur, and the power consumption is reduced.
However, since the absolute values of a set voltage Vset and a reset voltage Vreset in the respective structures of the unit cell are substantially equal to each other, the following problems occur during the operation in the cross-point structure.
In order to write data ‘0’ or ‘1’ into a selected cell, a write voltage larger than 2×Vreset or smaller than −2×Vreset should be applied across the selected cell. At this time, when the write voltage is applied to the selected cell, a voltage corresponding to ½ of the write voltage is applied to an unselected cell sharing a conductive line coupled to the selected cell in a cross-point structure. Since the voltage corresponding to ½ of the write voltage corresponds to a read voltage, a read operation of the unselected cell is performed.
In the CRS structure, when the read voltage is applied, the two structures included in the unit cell are changed to the LRS. In other words, during the read operation, the structure in the LRS in the unit cell storing data ‘0’ or ‘1’ maintains the state thereof, but the structure in the HRS is changed to the LRS. As a result, read-out is performed undesirably and changes the data stored in the unit cell. Therefore, during the write operation of the selected cell, the data stored in the unselected cell may be changed.